Among recent non-structural materials under intensive investigation there is a thermistor. The thermistor is a circuit element with its resistivity changing largely with temperature.
The thermistor is broken down into two types, NTC thermistor and PTC thermistor, in view of its temperature coefficient. The NTC thermistor is a material which has a negative temperature coefficient and whose resistivity decreases with increasing temperature, and is applied in the form of a temperature sensor to temperature compensation elements, etc. The PTC thermistor is on the other hand a material which has a positive temperature coefficient and whose resistivity increases sharply at a certain specific temperature. Taking this property, the PTC thermistor is widely applied to temperature control elements, overcurrent control elements, motor-starting elements, constant-temperature heat generators, etc.
A BaTiO.sub.3 base ceramic material is typical of such PTC thermistors. BaTiO.sub.3 is a ferroelectric material having a perovskite structure whose PTCR (positive temperature coefficient of resistivity) property becomes available at a temperature higher than its Curie temperature (Tc).
In this case, it is believed that the PTCR property becomes available through a potential barrier at a grain boundary. That is, excessive oxygen or impurities present on the surface of a crystal grain form an interface acceptor level, whereby an electron is captured from the crystal to form a depletion layer. At the grain boundary a Schottky potential barrier is formed. A high resistivity at a temperature higher than Tc is believed to be ascribed to this barrier. At temperatures less than Tc, on the other hand, spontaneous polarization causes a negative charge to be present at the grain boundary with a probability of 50%, so that the acceptor can be compensated for, resulting in a lowering of the barrier height. This is believed to contribute to a resistivity drop. At temperatures higher than Tc, the barrier height goes back to the original one due to the vanishing of spontaneous polarization. This is presumed to contribute to a resistivity rise.
In addition to BaTiO.sub.3 base ceramics, PbTiO.sub.3 --TiO.sub.2 base ceramics, too, are known as materials showing the PTCR property. A point common to these materials exhibiting the PTCR property is that they are ferroelectrics having a perovskite structure. It is further pointed out that important for these ferroelectrics is to make them semiconductive by a valence control method.
In JP-A 6-163204, the inventors have already proposed to obtain a semiconductive bismuth laminar oxide having the composition Bi.sub.4 Ti.sub.3 O.sub.12, with a part of Ti replaced by Nb, from bismuth laminar oxides which have a pseudo-perovskite type of subordinate lattices and most of which are ferroelectrics, and replacing a part of Bi in the obtained oxide by Sr, thereby imparting the PTCR property thereto. This bismuth laminar oxide is a good PTC thermistor material that can show the PTCR property and a sharp resistance change at temperatures exceeding 200.degree. C. In JP-A 6-283308, the inventors have also come up with a PTC thermistor material comprising a Bi.sub.2 O.sub.3 --TiO.sub.2 base composite oxide containing bismuth oxide and titanium oxide wherein a part of the titanium oxide has been replaced by at least one oxide selected from niobium oxide, tantalum oxide and antimony oxide and a part of the bismuth oxide has been replaced by at least one oxide selected from strontium oxide, calcium oxide and barium oxide.
A problem with these PTC thermistor materials is, however, that neither can the temperature at which the PTCR property becomes available be arbitrarily changed nor can the rate of resistivity change be arbitrarily controlled.
With the aforesaid PTC thermistor materials comprising composite oxides containing Bi and Ti, it is difficult to reduce their resistivity at a temperature lower than the temperature at which the PTCR property becomes available because the ceramics become semiconductive. When a PTC thermistor material is applied to a circuit part through which large currents pass in a normal state rather than in an overheated state, it is required to have a decreased resistivity at a temperature lower than the temperature at which the PTC property becomes available. However, a conventional semiconductive ceramic material can hardly be applied to such application. When it is somehow applied to such a circuit part, it is unavoidable that the size of the circuit part becomes excessively large.
In view of such situations as mentioned above, the inventors filed Japanese Patent Application No. 160746/1996 to come up with a PTC thermistor material having a structure wherein metal phases containing Bi as a main component are dispersed in an SrBi.sub.4 Ti.sub.4 O.sub.15 type phase. One great advantage of this PTC thermistor material is that it is possible to vary the melting point of the metal phases thereby controlling the temperature at which the PTCR property becomes available, because the availability of the PTCR property is determined by the melting of the metal phases. The electrical conductivity of the material at a temperature lower than the temperature at which the PTCR property becomes available is dependent on the size of the metal phases, the space factor of the metal phases in the material, etc.; that is, another advantage of the material is that its normal-temperature resistivity can be made lower than that of a conventional material by controlling such conditions. Thus, this PTC thermistor material can be applied to a circuit part through which large currents pass in a normal operation state.
For the fabrication of this PTC thermistor material, however, a firing atmosphere must be placed under relatively strict control, because it is required to precipitate metal phases from a matrix ceramic phase by firing in a reducing atmosphere. This, in turn, makes low-cost PTC thermistor material production difficult, and renders it difficult to mass produce PTC thermistor materials having stable properties. Further, the precipitation of the metal phases by reduction causes the metal phases to be substantially present only in the vicinity of the surface of the material. It is thus impossible to increase the ratio of the sectional area of a conducting path with respect to the sectional area of the material. In other words, when the material is designed as an element operated with an increased rated current, there is no choice all but to increase element size. Furthermore, since the zone of the PTC thermistor material where it can play its own role is restricted to the vicinity of the surface of the material, it is required to construct the material in the form of a sheet having a large specific surface area to decrease its volume. Such a limited degree of shape freedom imposes some considerable limitation on designing the material as an element.